2,6-Naphthalenedicarboxylic acid (2,6-NDA) is a monomer useful for the preparation of high performance polymeric materials such as polyesters and polyamides. Polyethylene 2,6-naphthalate (PEN) is one such high performance polyester and it is prepared, for example, by the condensation of either 2,6-naphthalenedicarboxylic acid or dimethyl-2,6-naphthalene-dicarboxylate with ethylene glycol. Fibers and films made from PEN have improved strength and thermal properties relative to, for example, fibers and films made from polyethylene terephthalate. High strength fibers made from PEN can be used to make tire cord, and films made from PEN are advantageously used to manufacture magnetic recording tape and electronic components. Also, because of its superior resistance to gas diffusion, and particularly to the diffusion of carbon dioxide, oxygen and water vapor, films made from PEN are useful for manufacturing food containers, particularly so-called "hot fill" type food containers. Polyesters made from mixtures of terephthalic acid and 2,6-naphthalenedicarboxylic acid or dimethyl-2,6-naphthalenedicarboxylate also have been found to have unique and desirable properties such as resistance to gas diffusion, making them suitable for manufacturing, for example, beverage containers or other containers for food products.
In order to prepare high quality polyesters suitable for the aforementioned applications, it is desirable to start with purified 2,6-naphthalenedicarboxylic acid or purified dimethyl-2,6-naphthalenedicarboxylate (DM-2,6-NDC). Since dimethyl-2,6-naphthalenedicarboxylate is typically prepared by the esterification of 2,6-naphthalenedicarboxylic acid using methanol, a purer form of 2,6-naphthalenedicarboxylic acid provides for purer dimethyl-2,6-naphthalenedicarboxylate. It is therefore advantageous to have the highest purity 2,6-naphthalenedicarboxylic acid.
2,6-Naphthalenedicarboxylic acid is most conveniently prepared by the liquid phase, heavy metal catalyzed oxidation of 2,6-dimethylnaphthalene using molecular oxygen, and particularly air, as the source of oxygen for the oxidation reaction. During this oxidation, the methyl substituents on the naphthalene ring of 2,6-dimethylnaphthalene are oxidized to carboxylic acid substituents. Processes for oxidizing 2,6-dimethylnaphthalene to 2,6-naphthalenedicarboxylic acid by such a liquid phase reaction are known. For example, U.S. Pat. No. 5,183,933 to Harper et al. discloses a continuous process for oxidizing 2,6-dimethylnaphthalene to 2,6-naphthalenedicarboxylic acid using high levels of manganese and cobalt oxidation catalyst metals added to the oxidation reaction mixture.
During the liquid phase oxidation of 2,6-dimethylnaphthalene to 2,6-naphthalenedicarboxylic acid using a catalyst comprising cobalt, manganese and bromine components various side products are usually produced. For example, trimellitic acid (TMLA) is produced by the oxidation of one of the rings of the 2,6-dimethylnaphthalene molecule. 2-Formyl-6-naphthoic acid (FNA), a result of incomplete oxidation of one of the methyl groups of the 2,6-dimethylnaphthalene molecule, is also produced. Bromination of the naphthalene ring during the oxidation reaction results in the formation of bromonaphthalenedicarboxylic acid (BrNDA). Additionally, loss of one methyl (or carboxylic acid) substituent during the oxidation reaction results in the formation of 2-naphthoic acid (2-NA). These side products, as well as a collection of other unidentified side products, are undesirable because they contaminate the 2,6-naphthalenedicarboxylic acid product.
We have also determined that when high levels of catalyst metals are used to oxidize 2,6-dimethylnaphthalene to 2,6-naphthalenedicarboxylic acid, these metals tend to remain with the 2,6-naphthalenedicarboxylic acid product making it difficult to purify the 2,6-naphthalenedicarboxylic acid in subsequent purification operations. For example, when 2,6-naphthalenedicarboxylic acid is esterified to produce dimethyl-2,6-naphthalenedicarboxylate, the residual catalyst metals in the 2,6-naphthalenedicarboxylic acid product foul heat exchangers and other equipment used to manufacture dimethyl-2,6-naphthalenedicarboxylate. Also, any catalyst metal removed in such esterification processes usually results in a loss of valuable product because the metals tend to remain complexed to or suspended in 2,6-naphthalenedicarboxylic acid or dimethyl-2,6-naphthalenedicarboxylate. Therefore, it is desirable to use low levels of oxidation catalyst metals not only from the standpoint of the cost of these catalysts, but also to reduce the complexity and expense of downstream purification procedures required to prepare sufficiently pure 2,6-naphthalenedicarboxylic acid or dimethyl-2,6-naphthalenedicarboxylate.
The art needs a process for the liquid-phase oxidation of 2,6-dimethylnaphthalene suitable for large-scale commercial operations that can produce 2,6-naphthalenedicarboxylic acid in high yield and having low levels of impurities, and which produces a product which is easily purified. The present invention provides such a process.